The invention relates to a hydraulic switching mechanism for longwall supports. Similar switching mechanisms are generally known and in use. They are hydraulic systems controlled by their own pressure. In these, the pump pressure of the hydraulic circuit is also used for hydraulic pilot control of the valves. This procedure has been successful in the control of supports. It allows the system to function with only two feed lines in the longwall. In contradistinction to this, in systems under external control, the hydraulic pilot control commands are generated via separate control valves operating independently of load pressure or pump pressure, the control valves being supplied with pressurizing medium via separate pressure lines. However, a part of any system of this type is also separate return flow management of all the control volumes. Also due to this, the complexity of the tubing is increased. In case faults appear, the localization of the fault is very difficult since it is not to be ruled out that the two pressure feeds, i.e. operating pressure feed and pilot control feed, interact. Also, the valve construction is significantly more complicated, in particular with regard to pressure equalization and sealing, due to the requirement of decoupling of the operating pressure from the pilot control pressure.
On the other hand, in view of the high level of complexity and the plurality of the switching elements and control elements located in the longwall, in particular control valves and load-maintaining valves, the high safety standards in mining entail a great expenditure in protective measures, even for hydraulic systems controlled by their own pressure. In this connection the basic rule applies that the hydraulic system for the shield support has to ensure that despite the large number of possible operating states, including impermissible operating states or unplanned faults, the personnel below ground can stay under the shields safely, even in the area of the longwall.
Despite these protective measures, it has been observed that in case of a fault in the operation of the pump, the ram (ram cylinder) of a shield, and in rapid temporal sequence also the ram cylinders of other shields, have slackened so that the rock mass could sag.
The objective of the invention is to develop hydraulic systems controlled by their own pressure and currently in use so that life-threatening and costly faults of this type cannot occur, and where the retrofitting of existing systems is also made possible without significant expenditure in modification.